The present disclosure generally relates to methods for stimulating a subterranean formation, and, more specifically, to methods for generating fluoride species that are reactive with a siliceous material.
Treatment fluids can be used in a variety of subterranean treatment operations. Such treatment operations can include, without limitation, drilling operations, stimulation operations, production operations, sand control treatments, and the like. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid or a component thereof, unless otherwise specified herein. Illustrative treatment operations can include, for example, drilling operations, fracturing operations, gravel packing operations, acidizing operations, scale dissolution and removal operations, sand control operations, consolidation operations, and the like.
Acidizing operations may be used to stimulate a subterranean formation to increase production of a hydrocarbon resource therefrom. During an acidizing operation, an acid-sensitive solid in the subterranean formation can be interacted with one or more acids to expand existing flow pathways in the subterranean formation, to create new flow pathways in the subterranean formation, and/or to remove precipitation damage in the subterranean formation. The acid-sensitive solid being interacted with the acid(s) can be part of the native formation matrix or can have been deliberately introduced into the subterranean formation in conjunction with a stimulation or like treatment operation (e.g., proppant or gravel particulates). Illustrative substances within the native formation matrix that may interact with an acid during an acidizing operation include, but are not limited to, carbonates, silicates and aluminosilicates. Other substances can also interact with an acid during the course of performing an acidizing operation, and the foregoing substances should not be considered to limit the scope of substances that may undergo acidization.
Carbonate formations can contain minerals that comprise a carbonate anion. Calcite (calcium carbonate) and dolomite (calcium magnesium carbonate) are representative examples. When acidizing a carbonate formation, the acidity of the treatment fluid alone can be sufficient to decompose the carbonate anion to carbon dioxide and leech a metal ion into the treatment fluid. Both mineral acids (e.g., hydrochloric acid) and organic acids (e.g., acetic and formic acids) can be used to treat a carbonate formation, often with similar degrees of success.
Siliceous formations can include minerals such as, for example, zeolites, clays, and feldspars, although various quantities of non-siliceous materials may also be present. As used herein, the term “siliceous” refers to a substance having the characteristics of silica, including silicates and/or aluminosilicates. Most sandstone formations, for example, contain about 40% to about 98% sand quartz particles (i.e., silica), bonded together by various amounts of cementing materials, which may be siliceous in nature (e.g., aluminosilicates or other silicates) or non-siliceous in nature (e.g., carbonates, such as calcite). Acidizing a siliceous formation or a formation containing a siliceous material is thought to be considerably different than acidizing a carbonate formation. Specifically, the mineral and organic acids that can be effective for acidizing a carbonate formation may have little effect on a siliceous formation, since these acids do not effectively react with siliceous materials. In contrast, hydrofluoric acid, another mineral acid, can react very readily with siliceous materials to promote their dissolution through forming a fluorinated silicon compound. Oftentimes, a mineral acid or an organic acid can be used in conjunction with hydrofluoric acid to maintain a low pH state within the treatment fluid as the hydrofluoric acid becomes spent during its reaction with the siliceous material. The low pH state may readily promote ready solubility of silicon or aluminum fluorides in the treatment fluid and aid in maintaining these substances in a dissolved state. The additional acid(s) may also promote dissolution of non-siliceous materials in the subterranean formation as well.
The introduction of hydraulic fracturing, horizontal drilling, and incursion into low permeability formations (e.g., shale formations) has greatly expanded the range of potential acidizing targets. Shale formations are considerably different from conventional hydrocarbon reservoirs. In this regard, they present a range of mineralogical analyses ranging from relative clean quartz (>90%) to mixtures with >20% clay. Carbonate minerals and sandstone may also be present. The other distinguishing feature of shale formations is their extremely low permeability, which may lie within the nanodarcy to microdarcy range. Due to capillary forces induced by the low permeability, fluid recovery from such formations can be a challenge, particularly aqueous fluid recovery. Although surfactants can be used to decrease the capillary forces and promote fluid recovery, their use can add to the cost and complexity of conducting a subterranean treatment operation.
Although treatment fluids containing hydrofluoric acid and, optionally, another acid can desirably affect dissolution of siliceous materials, the use of such low pH fluids can have unwanted consequences in certain instances. Specifically, at low pH values, particularly above about 2, dissolved fluoride ions can sometimes precipitate with aluminum and damage the subterranean formation, particularly in the presence of certain cations such as, for example, Group 1 metal ions (e.g., Na+ and K+) and/or Group 2 metal ions (e.g., Ca2+ and Ba2+). In some cases, precipitation of this type can damage a subterranean formation and inhibit production more than if the original treatment operation had not been performed at all. In certain cases, the problematic metal ions may be introduced to the subterranean formation through an aqueous carrier fluid in which the hydrofluoric acid and other acid(s) are present. Alkali metal acids such as, for example, KHF2 may also be problematic. In addition, aqueous carrier fluids containing one or more acids can present corrosion, safety, and waste disposal issues. Moreover, sourcing of an appropriate aqueous carrier fluid can also be an issue in certain instances, such as in remote or arid locations where water infrastructure may be limited.